This invention relates to devices for the treatment of heart disease and particularly to endo-arterial prostheses, which are commonly called stents. More particularly, the invention relates to stents incorporating flexible joints in the structure thereof that enable the stents to bend inside a patient""s vasculature.
Several interventional treatment modalities are presently used for heart disease, including balloon and laser angioplasty, atherectomy, and by-pass surgery. In typical coronary balloon angioplasty procedures, a guiding catheter having a distal tip is percutaneously introduced through the femoral artery and advanced into the cardiovascular system of a patient using a conventional Seldinger technique and advanced within the cardiovascular system until the distal tip of the guiding catheter is seated in the ostium of a coronary artery. A guide wire is positioned within an inner lumen of a dilatation catheter and then both are advanced through the guiding catheter to the distal end thereof. The guide wire is first advanced out of the distal end of the guiding catheter into the patient""s coronary vasculature until the distal end of the guide wire crosses a lesion to be dilated, then the dilatation catheter having an inflatable balloon on the distal portion thereof is advanced into the patient""s coronary anatomy over the previously introduced guide wire until the balloon of the dilatation catheter is properly positioned across the lesion. Once in position across the lesion, the balloon is inflated to compress the plaque of the lesion against the inside of the artery wall and to otherwise expand the inner lumen of the artery. The balloon is then deflated so that blood flow can be resumed through the dilated artery and the dilatation catheter can be removed therefrom. Further details of dilatation catheters, guide wires, and devices associated therewith for angioplasty procedures can be found in U.S. Pat. No. 4,323,071 (Simpson-Robert); U.S. Pat. No. 4,439,185 (Lindquist); U.S. Pat. No. 4,516,972 (Samson); U.S. Pat. No. 4,538,622 (Samson, et al.); U.S. Pat. No. 4,554,929 (Samson, et al.); U.S. Pat. No. 4,616,652 (Simpson); U.S. Pat. No. 4,638,805 (Powell); U.S. Pat. No. 4,748,982 (Horzewski, et al.); U.S. Pat. No. 5,507,768 (Lau, et al.); U.S. Pat. No. 5,451,233 (Yock); and U.S. Pat. No. 5,458,651 (Klemm, et al.), which are hereby incorporated herein in their entirety by reference thereto.
One problem that can occur during balloon angioplasty procedures is the formation of intimal flaps which can collapse and occlude the artery when the balloon is deflated at the end of the angioplasty procedure. Another problem characteristic of balloon angioplasty procedures is the large number of patients who are subject to restenosis in the treated artery. In the case of restenosis, the treated artery may again be subjected to balloon angioplasty or to other treatments such as by-pass surgery, if additional balloon angioplasty procedures are not warranted. However, in the event of a partial or total occlusion of a coronary artery by the collapse of a dissected arterial lining after the balloon is deflated, the patient may require immediate medical attention, particularly in the coronary arteries.
A focus of recent development work in the treatment of heart disease has been directed to endoprosthetic devices called stents. Stents are generally cylindrically shaped intravascular devices which are placed within an artery to hold it open. The device can be used to reduce the likelihood of restenosis and to maintain the patency of a blood vessel immediately after intravascular treatments. In some circumstances, they can also be used as the primary treatment device where they are expanded to dilate a stenosis and then left in place. Further details of stents can be found in U.S. Pat. No. 3,868,956 (Alfidi et al.); U.S. Pat. No. 4,512,338 (Balko et al.); U.S. Pat. No. 4,553,545 (Maass et al.); U.S. Pat. No. 4,733,665 (Palmaz); U.S. Pat. No. 4,762,128 (Rosenbluth); U.S. Pat. No. 4,800,882 (Gianturco); U.S. Pat. No. 4,856,516 (Hillstead); U.S. Pat. No. 4,886,062 (Wiktor); U.S. Pat. No. 5,421,955 (Lau); and U.S. Pat. No. 5,569,295 (Lam), which are hereby incorporated herein in their entirety by reference thereto.
One method and system developed for delivering stents to desired locations within the patient""s body lumen involves crimping a stent about an expandable member, such as a balloon on the distal end of a catheter, advancing the catheter through the patient""s vascular system until the stent is in the desired location within a blood vessel, and then inflating the expandable member on the catheter to expand the stent within the blood vessel. The expandable member is then deflated and the catheter withdrawn, leaving the expanded stent within the blood vessel, holding open the passageway thereof.
Advancing the stent through a patient""s vasculature, which can involve traversing sharp bends and other obstacles, may require the stent to be highly flexible. Stent flexibility also permits the stent to be deployed in and conform to a tortuous section of a patient""s vasculature. Additionally, visualizing the stent with a fluoroscope, which is currently the most widely used method of stent visualization during stent deployment, requires a stent with good radiopacity.
Different methods have been attempted to give stents high flexibility and radiopacity. By making stents out of relatively thin material, flexibility can be increased. However, the use of thin material can reduce the radiopacity of the stent, which can make it difficult for a physician or technician to visualize the stent. Conversely, the use of thicker material, which promotes radiopacity, can reduce stent flexibility and resultantly impair the deliverability of the stent.
An early attempt at achieving a flexible stent with good radiopacity characteristics involved providing a stent of a base material with good flexibility and strength but relatively low radiopacity, and then adding a thin layer of a highly-radiopaque material, such as gold, to the stent. This approach, which required the use of two separate materials, involved a relatively complicated process in applying the radiopaque material to the stent. Additionally, the use of multiple materials can complicate use and deployment of the stent, particularly where the different materials have different material characteristics, such as different strengths, different biocompatibility, or different responses to temperature changes.
Another approach was to provide a stent with substantially thicker portions at each end. Such an approach provided a stent with highly radiopaque ends, so that a physician could easily view the stent ends during stent delivery.
What has been needed and heretofore unavailable is an improved means of providing a stent with high flexibility, strength, and radiopacity. The present invention satisfies this need.
This invention relates to devices for the treatment of heart disease and particularly to endo-arterial prostheses, which are commonly called stents. More particularly, the invention relates to stents incorporating flexible joints in the structure thereof enabling the stents to easily bend to conform to a patient""s vasculature.
Present day expandable stent designs incorporate portions that flex or otherwise deform during insertion of the stent into a patient""s vasculature. Similarly, there are also portions of the stent that remain more stable (i.e., less deformed) during insertion.
It will be appreciated that as a stent advanced along a circuitous path in a coronary artery, it flexes about it longitudinal axis in order to navigate curves in the patient""s vasculature. In this invention, when the stent is flexed while passing through the bends in an artery, some portions of the stent will flex substantially while other portions, remain less deformed. Thus, the stent has highly flexible portions and more stable (i.e., less deformed) portions that function accordingly during insertion into a patient""s vasculature.
The flexibility of a stent upon insertion into a patient""s vasculature is largely derived from highly flexible portions of the stent, without substantial assistance from the more rigid or stable portions. Stable portions in this case are those portions that, due to the overall stent design, will, when implanted at the implant site resiliently assume their distended circumferential configuration during insertion. Some of these portions are configured to flex under selected loads such as experienced during implantation. These stable portions provide strength to afford support to resist collapse from radially inwardly acting forces, but do not substantially flex or otherwise deform while the stent is inserted through the vasculature.
What makes the stent of the present invention highly flexible is the incorporation of flexible joints which couple the cylindrical rings to each other. The flexible joints, as described below are designed to be flexible and enable the stent, to, while being advanced to the implant site, flex significantly about the longitudinal axis. The resulting flexible stent can be more safely advanced through a patient""s vasculature during delivery.
The stent of the present invention generally includes a plurality of radially expandable cylindrical rings which are relatively independent of one another in their ability to expand and flex relative to one another. Flexible joints enable the rings to behave accordingly. The flexible joints may be formed by the opposite extremities of connecting links joining adjacent cylindrical rings together. The links provide increased stability and help to prevent warping of the stent upon expansion and the springs enable the stent to flex to conform to a tortuous vessel. Either or both of the springs and links can be formed with smaller cross-sections or be formed of a selected material more flexible than the remainder of the cylindrical rings to enhance flexibility.
The resulting stent structure is a series of radially expandable cylindrical rings spaced longitudinally apart and connected to each other by the flexible joints formed by links and springs. Due to the flexibility of the springs which couple the links to the rings, the individual rings may rotate slightly relative to each other without significant deformation. Cumulatively, the stent is flexible along its length and about its longitudinal axis but is still relatively stiff in the radial direction in order to resist collapse once implanted.
One preferred structure for the expandable cylindrical rings is a generally circumferential undulating pattern, e.g., serpentine. The cylindrical rings typically are formed of a plurality of undulations defining peaks and valleys, where the valleys of one cylindrical ring can be circumferentially offset from the valleys of an adjacent cylindrical ring. In this configuration, at least one link attaches each cylindrical ring to an adjacent cylindrical ring so that one end of a link is positioned substantially within one of the valleys and the other end attaches the valley to a peak of an adjacent ring. When the rings are not circumferencially offset a valley can be connected through the flexible joint to a valley in an adjacent ring. In this configuration, both the peaks and valleys of adjacent rings will be longitudinally aligned with each other.
The rings themselves are formed by a series of U and/or W-shaped undulations disposed in a generally elongated cylindrical configuration and arranged in repeating patterns. While the cylindrical rings are not divided up or segmented into U""s and/or W""s, the pattern of the cylindrical rings resemble such a configuration. The longitudinal U and W patterns promote some flexibility in the stent by flexing and by tipping radially outwardly at one end or the other as the stent is flexed to curve in one direction or the other along its longitudinal axis as it is advanced along in a tortuous vessel. The flexibility during insertion though is generally limited due to the thickness required in the ring to afford resistance to radial collapse and present adequate radiopacity.
In order to balance the need for increased stent thickness driven by the radiopacity requirements mentioned above, with the need for flexibility/deployment requirements, the current invention provides a stent having flexible joints which include springs formed with undulations defined by smaller, sub peaks and valleys. These springs can couple the connecting links to the rings to allow for greater freedom of ring conformability. The links also can be connected at their respective one or both ends directly to the respective rings for structural rigidity purposes. Furthermore, each ring is generally formed with more than one spring so as to be interposed between segments of the respective rings. The springs are configured to provide for the adjacent cylindrical rings flexing and compressing with respect to each other to increase the stent""s ability to flex and bend around tight or tortuous anatomy.
The rings are configured so the springs cooperate to enable the stent to flex longitudinally during and after stent expansion in order to conform to the vessel wall. This conformability has been shown though experimentation to reduce the likelihood of restenosis so when compared to stents which don""t conform to the same degree. The springs cooperate to cause such rings to substantially retain their pre-insertion dimensions during and after insertion so as to retain their flexible characteristics.
The stent design of the current invention resists the tendency of twisting at one end of the stent to be transmitted along the length of the stent while it is being expanded in a vessel. Coupling the links to the rings at the respective springs serves to provide for lost rotary motion between adjacent ones of the rings. That is, as the stent is navigated through the circuitous vasculature, rotary forces may be applied to a ring at one end of the stent causing it to rotate slightly in one direction. The flexibility in the respective springs will result in such spring flexing slightly allowing the respective connecting link to angle in one direction or the other thus minimizing the tendency of such rotary force to be transmitted on to the adjacent ring. This lack of rotational translation from ring to ring serves to minimize the tendency of rotary forces applied to one end of the stent resulting in a bodily rotation of the entire stent and the resultant loss of control during placement at the implant site.
The sub peak and valley segments forming the springs can also comprise a series of U and/or W-shaped structures. While not divided up or segmented into U and/or W segments, the pattern of the sub peaks and valleys resemble such a configuration. The flexibility is generally high because the transverse cross-sections of the springs are generally smaller than those of undulations in the rings. Such cross-sections may be small because there is generally less need for radiopaque properties, it being recognized that the cross-sections of the undulations can be larger so as to cooperate in meeting the radiopaque requirements discussed previously.
The stent embodying features of the invention can be readily delivered to the desired lumenal location by mounting it on an expandable member of a delivery catheter, for example a balloon, and passing the catheter-stent assembly through a body lumen to an implantation site. A variety of means for securing the stent to the expandable member on the catheter are available.
It will be appreciated that, in the preferred embodiment the expandable rings are coupled together to cooperate in forming an outline of an expandable cylindrical shell. Radial expansion of such shell may be achieved in the undulating pattern expanding its waveform to, in effect, decrease the waveform""s amplitude and the frequency. The rings are so constructed as to be expanded to be plastically deformed (except with NiTi alloys) so that the stent itself will remain in its expanded condition and have sufficient rigidity to support the center of the vessel against collapse. During expansion of the stent, the respective one end or the other of the undulating portion of the rings may tip outwardly to extend radially outwardly from the exterior cylindrical outline defined by the expanded stent, when in its unflexed condition to thus slightly radially embed in the wall of the vessel and help secure the expanded stent in its implanted location.
In those embodiments when the stent is constructed of superelastic NiTi alloys, the expansion occurs when the stress of compression is removed so as to allow the phase transformation from martensite back to austenite and as a result the expansion of the stent.
The links which interconnect adjacent cylindrical rings may be constructed with relatively small transverse cross-sections similar to the transverse dimensions of the undulating components of the expandable cylindrical elements and, in other embodiments, may be thinner to further facilitate flexibility. The particular thickness of the links depends on, among other factors, flexibility goals as well as rigidity requirements. The links may be formed in a unitary structure with the expandable cylindrical elements from the same intermediate product, such as a tubular element, or they may be formed independently and connected by suitable means, such as by welding or by mechanically securing the ends of the links to the ends of the expandable cylindrical elements.
The springs which connect the opposite ends of the links joining adjacent cylindrical rings may also have a transverse cross-section similar to the transverse dimensions of the undulating components of the expandable cylindrical elements in some embodiments, while in other embodiments, the cross-section may be thinner. The springs may be formed in a unitary structure with the expandable cylindrical elements from the same intermediate product, such as a tubular element, or they may be formed independently and connected by suitable means, such as by welding or by mechanically securing the ends of the interconnecting elements to the ends of the expandable cylindrical elements. In expansion the springs do not expand similarly to the undulations comprising peaks and valleys. Rather, the springs, comprised of subpeaks and valleys, substantially retain their original shape so as to continue to provide flexibility in the stent after implantation.
The number and location of the springs can be varied in order to develop the desired longitudinal as well as rotational flexibility in the stent structure both in the unexpanded as well as the expanded condition. These properties are important to minimize alteration of the natural physiology of the body lumen into which the stent is implanted and to maintain the compliance of the body lumen which is internally supported by the stent. Generally, the greater the longitudinal flexibility of the stent, the easier and the more safely it can be delivered to the implantation site. The number of links can also be varied in order to tailor the stent to specific applications.
The stent may be formed from a tube by laser cutting the pattern of cylindrical rings and links in the tube. The stent may also be formed by laser cutting a flat metal sheet in the pattern of the cylindrical rings and links, and then rolling the pattern into the shape of the tubular stent and providing a longitudinal weld to form the stent.